1. Field of the Invention:
This invention relates to a printer, in particular, to a dot matrix printer for a computer, the dot matrix printer having a print head which is movable on a guide along the print line. The distance between the print head and the printing bed plate is adjustable by means of a spacing adjustment apparatus so that printing can be done on single or multiple charts which are lying on the bed plate and which may be of uneven thickness. A sensing roller is rotatably mounted on the print head to contact and roll along a chart and/or the bed plate to measure the thickness of the material being printed on.
2. Background Information:
One purpose of printers having distance adjustment devices for the print head is for the handling of different types and thicknesses of documents, e.g., savings account passbooks, or possibly even thicker documents, such as charts, or even books. Another purpose for the distance adjustment apparatus, however, is for setting the correct distance between the printing elements, which have a limited stroke, and the surface being printed upon, thereby producing the most optimum print possible. As an example, the stroke distance of typical printer pins, which stroke distance is the maximum distance a pin will move from the printing head, may be only approximately 0.3 mm to 0.5 mm. Thus, if the distance between the print head and the surface being printed upon were greater than the 0.3 mm to 0.5 mm stroke distance, no printing would occur, or if the distance was set very close to the 0.3 mm to 0.5 mm limit, the print produced would ultimately end up very light.
There are numerous examples in the prior art of printers having distance adjustment devices which operate on the basis of a spring-mounted sensing roller. One of the examples of the prior art, which is disclosed in German Publication No. DE-C1 36 41 044, is a separate rocker, or oscillating arm on which the sensing roller is mounted, and an essentially parallel rocker arm on which the print head is mounted. In this application, the two rocker arms move relative to one another under pressure applied by a spring.
An additional example of the prior art, which is disclosed in German Publication No. DE-Al 38 30 880, is a sensing apparatus in which the movement of the sensing roller, which is spring-mounted, is transmitted to a sensor and an indicator via a signal amplifier, and from there, via an analog-digital converter to a microprocessor. A stepper motor is thereby digitally controlled so that the stepper motor drives a gear wheel segment which is connected to a cam on which the print head carriage is movably mounted.
Both of these prior art solutions have at least the following two things in common, that is, first, they both have a spring-mounted sensing roller, and second, they both require that the chart being printed on be of uniform thickness over the entire print path. In each of these applications, the starting edge of the chart, among other things, need not necessarily be measured for the determination of the width of the chart as such a measurement is unnecessary.